AWS IAM: how do I give my EC2 app access to S3 without hardcoding keys?

Question

My app on EC2 has `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY` in a `.env` file. A colleague said that's bad practice. What's the alternative?

Accepted Answer

Use an **IAM role attached to the instance** (an *instance profile*). The AWS SDKs automatically fetch **temporary, auto-rotating credentials** from the instance metadata service, so there are no keys to leak or rotate.

1. Create a role with the EC2 trust policy.
2. Attach a **least-privilege** policy:

```json
{
  "Version": "2012-10-17",
  "Statement": [{
    "Effect": "Allow",
    "Action": ["s3:GetObject", "s3:PutObject"],
    "Resource": "arn:aws:s3:::my-app-uploads/*"
  }]
}
```

3. Attach the role to the instance and **delete the keys** from `.env` (and deactivate them in IAM).

The SDK code doesn't change: `new S3Client({})` finds the role credentials itself. The same idea applies elsewhere: ECS task roles, Lambda execution roles, EKS IRSA/Pod Identity. Also enforce **IMDSv2** on the instance.

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